Studies on the origin of pandemic influenza viruses in man are aimed at elucidating the mechanism whereby new human strains occur. In vivo studies in animals show that recombination can occur between the influenza A viruses of man, lower animals, and birds under natural conditions of transmission. These viruses are "new" in that they are hybrids possessing properties of both parental viruses and are genetically stable. Studies on influenza viruses from horses (equine 2) and ducks (duck/Ukraine/63) indicate that the hemagglutinin sbunits of these viruses are related immunologically and biochemically to the hemagglutinin of Hong Kong influenza virus. These studies indicate the importance of the influenza viruses from lower animals and birds in the emergence of new pandemic strains and that recombination is a possible mechanism for the origin of these viruses.